This invention relates to power semiconductor devices, and more particularly to Schottky barrier rectifiers.
Schottky barrier rectifiers are used extensively as output rectifiers in switching-mode power supplies and in other high-speed power switching applications, such as motor controls, for carrying large forward currents and supporting large reverse blocking voltages. As is well known to those having skill in the art, rectifiers exhibit low resistance to current flow in a forward direction and a very high resistance to current flow in a reverse direction. As is also well known to those having skill in the art, a Schottky barrier rectifier produces rectification as a result of nonlinear unipolar current transport across a metal semiconductor contact. A detailed discussion of the design of Schottky barrier power rectifiers may be found in Chapter 4 of a textbook by B. J. Baliga entitled xe2x80x9cPower Semiconductor Devices,xe2x80x9d PWS Publishing Co. (1995), the disclosure of which is hereby incorporated herein by reference.
SiC Schottky rectifiers are a promising technology since they offer the potential for a low forward voltage drop, high breakdown voltage, and fast switching speed with no reverse recovery current, as discussed in the following papers which are hereby incorporated by reference: M. Bhatnagar et al., xe2x80x9cSilicon-Carbide High-Voltage (400 V) Schottky Barrier Diodes,xe2x80x9d IEEE Electron Device Letters, Vol. 13, No. 10, October 1992, pp. 501-503; and A. Itoh et al, xe2x80x9cExcellent Reverse Blocking Characteristics of High-Voltage 4Hxe2x80x94SiC Schottky Rectifiers with Boron-Implanted Edge Termination,xe2x80x9d IEEE Electron Device Letters, Vol. 17, No. 3, March 1996, pp. 139-141. However, the design of a power Schottky rectifier requires a tradeoff in selecting the optimal Schottky metal. The power dissipated by a Schottky rectifier depends on the forward voltage drop and the reverse leakage current, both of which should be as low as possible, and both of which are related to the Schottky barrier height, i.e., the magnitude of the potential barrier between the metal and semiconductor regions of a Schottky contact. A small barrier height metal will have a low forward voltage drop and a large reverse leakage current. Conversely, a large barrier height metal will have a larger forward voltage drop and a smaller reverse leakage current. Therefore, it is desirable to have a Schottky rectifier which exhibits the forward characteristics of a small barrier height metal and the reverse characteristics of a large barrier height metal. These conflicting design criteria have heretofore not been met in materials such as SiC. The problem of conflicting design criteria is compounded by the fact that SiC Schottky rectifier reverse leakage currents have been observed to be larger than predicted by thermionic emission theory. See, e.g., M. Bhatnagar et al., xe2x80x9cEffect of Surface Inhomogeneties on the Electrical Characteristics of SiC Schottky Contacts,xe2x80x9d IEEE Transactions on Electron Devices, Vol. 43, No. 1, January 1996, pp. 150-156, which paper is hereby incorporated by reference.
One proposed solution to the problem is to use a xe2x80x9cpinchxe2x80x9d Schottky rectifier. A pinch Schottky rectifier utilizes a high barrier region to pinch-off or electrically shield a low barrier region. Many different designs of pinch rectifiers have been implemented in Si, including implanted/diffused P-N junction pinch rectifiers (junction barrier Schottky (JBS) rectifiers), trench-JBS (TJBS) rectifiers, and trench-MOS barrier-Schottky (TMBS) pinch rectifiers, discussed respectively in the following papers which are hereby incorporated by reference: B. J. Baliga, xe2x80x9cThe Pinch Rectifier: A Low-Forward-Drop High-Speed Power Diode,xe2x80x9d IEEE Electron Device Letters, Vol. 5, No. 6, June 1984, pp. 194-196; S. Kunori et al., xe2x80x9cThe Lower Power Dissipation Schottky Barrier Diode with Trench Structure,xe2x80x9d Proceedings of 1992 International Symposium on Power Semiconductor Devices and ICs, Tokyo, pp. 66-71; and M. Mehrotra et al., xe2x80x9cTrench MOS Barrier Schottky (TMBS) Rectifier: A Schottky Rectifier with Higher than Parallel Plane Breakdown Voltage,xe2x80x9d Solid-State Electronics, Vol. 38, No. 4, 1995, pp. 801-806. In a paper by B. J. Baliga entitled xe2x80x9cTrends in Power Semiconductor Devicesxe2x80x9d and published in IEEE Transactions on Electron Devices, Vol. 43, No. 10, October 1996, pp. 1717-1731, which paper is hereby incorporated by reference, JBS and TMBS rectifiers have been recognized as evolutionary improvements to the planar power Schottky rectifier device structure.
The implementation of a JBS or TJBS in SiC is somewhat difficult due to the lack of diffusion of dopant impurities in SiC. In addition, due to the large critical field of SiC, the usefulness of a SiC TMBS would be somewhat limited by oxide breakdown.
It has also been proposed to achieve a more favorable tradeoff between forward voltage drop and reverse leakage current by providing regions of different barrier heights in a Schottky barrier rectifier. U.S. Pat. No. 5,262,668 to Tu et al. discloses such a device implemented in silicon. There is no apparent recognition of the limited benefits obtained with a silicon substrate and no suggestion that SiC would be preferable as a substrate material for any reason.
Despite all the developments in the state of the art, there remains a need for improvements in SiC Schottky barrier rectifiers over known devices in terms of low forward voltage drop and low reverse leakage current, resulting in a significant reduction in total power dissipation.
The present invention meets this need and others with a dual-metal-trench (DMT) SiC Schottky pinch rectifier. The device has a plurality of trenches formed in a SiC substrate of a first conductivity type, and includes a first Schottky contact at the bottom of each trench and a second Schottky contact on each mesa between adjacent ones of the trenches. The first Schottky contact has a relatively high barrier height, and the second Schottky contact has a relatively low barrier height.
Another aspect of the present invention is a simplified method of fabricating a SiC Schottky rectifier. The method includes the following steps: forming a bi-layer metal pattern, forming a bi-layer metal pattern on a silicon carbide substrate, the bi-layer metal pattern including a lower layer in contact with the silicon carbide substrate and an upper layer having the same length and width as the lower layer, wherein the lower layer is a metal having a relatively low barrier height and the upper layer is suitable as an etch mask; forming a pattern of trenches in the silicon carbide substrate by reactive ion etching with the upper layer of the bi-layer metal pattern as an etch mask, whereby the lower layer metal is self-aligned to the trenches; forming a metal layer in the trenches; and forming an ohmic contact on the bottom surface of the substrate.
It is a general object of the present invention to provide improvements in the performance of power SiC Schottky rectifiers.
A more specific object of the invention is to provide a SiC Schottky rectifier having forward bias characteristics similar to those of a SiC Schottky rectifier with a low barrier height, and reverse bias characteristics similar to those of a SiC Schottky rectifier with a high barrier height.
A further object of the present invention is to provide a relatively simple process of fabricating a SiC Schottky rectifier.
These and other objects and advantages of the present invention will be more apparent upon reading the following detailed description of the preferred embodiment in conjunction with the accompanying drawings.